1. Field of the Invention
The present invention is in the field of chemical cooking of wood chips to produce pulp suitable for manufacture of paper and involves sequential use of various cooking liquors in the same digester for predetermined cooking times, to increase the efficiency of the process and to conserve energy.
2. Description of the Prior Art
There are numerous types of processes for batch digestion of wood chips in the manufacture of paper. The digestion usually takes place in a digester specifically built for that purpose, the digester being filled with the wood chips which are usually compacted therein. Hot solutions of sodium hydroxide alone or in admixture with sodium sulfide are then charged into the digester. The temperature of the digester can be controlled through the introduction of steam and after maintaining the chips in contact with the cooking liquor for a predetermined period of time, a blow valve in the digester can be opened to dump the contents into a blow tank.
There is a substantial amount of heat loss in carrying out the batch digestion process and while many systems have been suggested for minimizing this heat loss, none has been particularly effective. Some paper manufacturers have gone to continuous digesting processes in order to improve the efficiency of the cooking operation, but the equipment costs for a continuous digesting system are very high.
More recently, an improved type of batch process has been designed for overcoming these difficulties. In this new process, the wood chips are cooked in the digester and the hot black liquor which results is removed by displacement with a filtrate from the washing section. This filtrate is added to the bottom of the digester and pushes up the hot spent liquor through the chip column without a substantial intermingling of the two liquids. The displaced hot black liquor is then directed into a pressurized accumulator. The digester is then emptied by adding steam to the top of the digester which forces the pulp out through a blow valve into a blow tank. After the pulp has been blown from the digester, it is uniformly filled with chips.
Hot black liquor from the accumulator is pumped into the bottom of the digester where it heats the chips. In this stage, an excess of black liquor is employed, more than the capacity of the digester so that excess black liquor is discharged from the top of the digester and is transferred to a weak black liquor tank. Fresh white liquor is then used to displace the black liquor from the bottom of the digester and the resulting spent liquor is passed to a weak black liquor storage space. The contents of the digester are then heated with steam to the desired cooking temperatures and held there for the required cooking times. When the contents of the digester have reached the cooking temperature, the steam introduction stops. After cooking, the hot liquor is removed as in the originally described step, and the cycle starts over again. This type of process is described in Fagerlund Application Ser. No. 434,758, filed Oct. 18, 1982, now U.S. Pat. No. 4,578,149 and assigned to the assignee of the present application.